The Cell Phone is a key item that is always in Kris's inventory. It can be used by selecting "CELL" and a listed number while in Hometown. Its use causes unique dialogue depending on the Chapter and Kris's location.
At the beginning, the phone only has Toriel's home phone. Calling this number yields different dialogue from Toriel depending on the Chapter and Kris's location:
- If called from Toriel's House, Toriel picks up but becomes agitated with Kris.
- If called from the main room, the landline rings. However, narration jokingly states that Kris cannot pick up the phone, as they are already on their cell phone.
- In Chapter 1, after Kris returns from the Card Kingdom, they call Toriel on the way out of School. Toriel expresses worry, but is happy that Kris was spending the time with "a friend." She permits Kris to stay out a little longer.
- If called again while in Hometown, Kris asks Toriel to drive them home. Toriel instead encourages them to walk home and reminds them of where her house is located. On the third call, she mentions she is busy grading.
- In Chapter 2, Kris can call Toriel's cell phone while they have a "trash orb" at the School's southern hallway. They have a slightly different response depending if they overheard Alphys and Toriel. It is implied Kris attempts to offer their phone to Alphys so she can explain their response. Immediately after, Kris informs Toriel that they will be at the Library.
- If they did not overhear, they inform Toriel that they "made a "trash orb"."
- If Kris calls after they overhear, they state they are "normal."
- If called a second time after either of these, Toriel states to not worry about the library.
- If called before entering Cyber World instead, Kris notifies Toriel that they are going to work on their project. Toriel expresses excitement to help before Kris states that they have a partner. Toriel is elated, and hopes that their partner likes homemade, "butterscotch-scented gluesticks."
- After any of the above calls, Toriel comments to not worry about the library. She states that if Berdly pesters Kris over their debt, she may come and throw a "smoke bomb" so Kris can escape. She laughs at her joke, and encourages Kris to try their best before hanging up.
- If called after returning from Cyber World, Toriel asks Kris how their school project went and plans to drive them home. Kris states that there is no need, though Toriel but is happy to hear they are spending time with their friend. She tells them to be home by sunset before hanging up.
- If Kris calls a second time, Toriel is amused and questions Kris, "What must your friend think?" She then ends the call by telling Kris to have fun with their friend.
- If called while Toriel and Susie are in the kitchen, Toriel asks Kris to pick up the landline phone. She becomes annoyed and confused when she realizes Kris called.
- If Kris calls while in the bathroom instead, they ask Toriel to give the phone to Susie. Susie yells to Kris to hurry up and finish washing their hands.
- After either of the above calls: "(Everybody seems too busy to pick up the phone...)"
- If used in the Dark World, the Cell Phone emits 6 seconds of an unskippable
mus_smile.ogg, and does not function.
- If used in Chapter 2, the secondary text is altered. "It's nothing but garbage noise."
- By talking to Sans, Kris can obtain a second phone number. However, when called, it directs Kris to a hotline for idiot babies, where the player can choose if they are an Idiot, a Baby, an Idiot Baby, or None. The option then changes to "Not Sans's Number" and rings indefinitely when called.
- This number cannot be called in Chapter 2, as it is no longer listed in the "CELL" option.
mus_smile.oggis the same sound that plays during W. D. Gaster's entry number seventeen. It is an inaccessible room titled Room 264 in Undertale.
- In Chapter 1, the call after Kris returns from Card Kingdom is scripted to occur at the School entrance. If Kris calls before then, the dialogue for the first call is the same.
- The debt Toriel is aware of is for the "How to Draw Dragons" book that has been overdue for over 2583 days. It is implied through the narration that Asriel will never return the book.
- The Chapter 2 narration after using the Cell Phone is similar to an Addison's description when they searched for Spamton.
- (...) (Your brother will never return this book.) - Narration upon inspecting the second floor's hallway drawer in Chapter 2.
- He must've left in the middle of a conversation, Because I could still hear someone on the other end... But when I put the receiver to my ear... There was nothing but garbage noise. - Addison, Chapter 2.